A seismic survey represents an attempt to image or map the subsurface of the earth by sending energy down into the ground and recording the “echoes” that return from the rock layers below. The source of the down-going sound energy might come, for example, from explosions or seismic vibrators on land, or air guns in marine environments. During a seismic survey, the energy source is placed at various locations near the surface of the earth above a geologic structure of interest. Each time the source is activated, it generates a seismic signal that travels downward through the earth, is reflected, and, upon its return, is recorded at a great many locations on the surface. Multiple source/recording combinations are then combined to create a near continuous profile of the subsurface that can extend for many miles. In a two-dimensional (2D) seismic survey, the recording locations are generally laid out along a single line, whereas in a three dimensional (3D) survey the recording locations are distributed across the surface in a grid pattern. In simplest terms, a 2D seismic line can be thought of as giving a cross sectional picture (vertical slice) of the earth layers as they exist directly beneath the recording locations. A 3D survey produces a data “cube” or volume that is, at least conceptually, a 3D picture of the subsurface that lies beneath the survey area. In reality, though, both 2D and 3D surveys interrogate some volume of earth lying beneath the area covered by the survey.
A conventional seismic survey is composed of a very large number of individual seismic recordings or traces. In a typical 2D survey, there will usually be several tens of thousands of traces, whereas in a 3D survey the number of individual traces may run into the multiple millions of traces. Chapter 1, pages 9-89, of Seismic Data Processing by Ozdogan Yilmaz, Society of Exploration Geophysicists, 1987, contains general information relating to conventional 2D processing and that disclosure is incorporated herein by reference. General background information pertaining to 3D data acquisition and processing may be found in Chapter 6, pages 384-427, of Yilmaz, the disclosure of which is also incorporated herein by reference.
A seismic trace is a digital recording of the acoustic energy reflecting from inhomogeneities or discontinuities in the subsurface, a partial reflection occurring each time there is a change in the elastic properties of the subsurface materials. The digital samples are usually acquired at 0.002 second (2 millisecond or “ms”) intervals, although 4 millisecond and 1 millisecond sampling intervals are also common. Each discrete sample in a conventional digital seismic trace is associated with a travel time, and in the case of reflected energy, a two-way travel time from the source to the reflector and back to the surface again, assuming, of course, that the source and receiver are both located on the surface.
Many variations of the conventional source-receiver arrangement are used in practice, e.g. VSP (vertical seismic profile) surveys, ocean bottom surveys, etc. Further, the surface location of every trace in a seismic survey is carefully tracked and is generally made a part of the trace itself (as part of the trace header information). This allows the seismic information contained within the traces to be later correlated with specific surface and subsurface locations, thereby providing a means for posting and contouring seismic data—and attributes extracted therefrom—on a map (i.e., “mapping”).
Of particular interest for purposes of the instant application are seismic exploration techniques such as VSPs or similar technology. By way of general background, a VSP survey is an exploration technique in which a seismic signal is generated at or near the surface and subsequently sensed by one or more geophones (land seismic sensors) or hydrophones (marine seismic sensors) that are situated in the subsurface, e.g., within a cased or uncased well which may or may not have been drilled for that purpose.
VSP seismic data are often used to support and clarify the subsurface interpretation obtained from other seismic data sources (e.g., conventional surface seismic, well logs, cores, etc.). Because the VSP receivers are situated in the subsurface they potentially yield unique information about the up going and down going seismic energy and, since they are located much nearer to the subsurface target(s) of interest (and, in more particular, are located below the surface weathering layer) than surface receivers, there is an expectation that the data collected thereby will be yield a more representative image of the subsurface.
Related in general concept to the VSP survey is a checkshot survey, which also utilized a surface source and downhole receivers (e.g., seismic receivers that are positioned within a producing well, a well that is being drilled, a well that was created for purposes of seismic imaging, etc.). However, the checkshot survey is directed not so much toward imaging the subsurface, but rather toward development of a velocity profile in the rocks near the well. One difference between a VSP survey and a checkshot survey is that in a checkshot survey attention is typically directed only toward the first breaks (earliest arrivals) of the seismic energy from the source, whereas in a VSP survey it is the seismic energy that is sensed following the first break that is most useful for purposes of seismic imaging. Of course, those of ordinary skill in the art will understand that a VSP survey also yields a checkshot survey, but not vice versa. Finally, the various methods of collecting and processing VSP and checkshot data to make them useful in seismic exploration are well known to those of ordinary skill in the art and, as such, will not be covered herein.
One persistent problem that tends to limit the effectiveness of seismic in some locations is the challenge of obtaining good images near complex structures such as salt domes. In such cases, it is customary to apply imaging processes such as migration to the seismic data to relocate the observed reflectors in time and/or space, thereby causing them to more accurately represent the actual subsurface structural configuration. However, the ability to perform accurate time or depth migrations depends heavily on having knowledge of the subsurface rock properties (including, the velocity at every subsurface point), which information may be imperfect or lacking. In such cases, it is customary to estimate (e.g., from velocity spectra) the velocities using seismic data or, where it is available, well logs, etc.
More generally, seismic migration is the process by which wavefields recorded on or near the surface are mapped back into the sub-surface to form an image of the sub-surface geology (structure). Common to modern migration methods is that they typically rely on a computational model for the subsurface, so that approximate Green's functions can be constructed between all surface (recording) locations and all image points in the sub-surface. Those of ordinary skill in the art will recognize that the accuracy and complexity of this computational model will determine the image fidelity it is possible to achieve. Most state-of-the-art migration methods use Green's functions that include multiple arrivals and some finite-frequency effects, allowing them to image fairly complex geological structures. However, the computational model is in most cases only represented as a scalar wave speed (velocity) with smooth spatial variations. Some models will also include certain discontinuities in the velocity in an effort to represent geological boundaries that separate regions with different velocity. Whatever the model is, it has to be derived from measurements, and it will at best be a non-unique solution to an inverse problem. Commonly, much more crude approximations are used, say, that only try to match the predicted first-arrivals to the observed first arrivals in the data.
What is needed, then, is a method of calculating Green's functions for use in seismic imaging that does not suffer from the disadvantages of the prior art. Additionally, the method should provide for true amplitude (or near true amplitude) processing.
Heretofore, as is well known in the geophysical prospecting and interpretation arts, there has been a need for a method of using seismic data to obtain image of the subsurface that does not suffer from the limitations of the prior art. Accordingly, it should now be recognized, as was recognized by the present inventors, that there exists, and has existed for some time, a very real need for a method of geophysical prospecting that would address and solve the above-described problems.
Before proceeding to a description of the present invention, however, it should be noted and remembered that the description of the invention which follows, together with the accompanying drawings, should not be construed as limiting the invention to the examples (or preferred embodiments) shown and described. This is so because those skilled in the art to which the invention pertains will be able to devise other forms of this invention within the ambit of the appended claims.